1. Field of the Invention
The present invention relates to a multiple resonator and variable-wavelength light source used for an optical multiplexing transmission system such as a WDM (Wavelength Division Multiplexing) transmission system, and more particularly, to a multiple resonator having parameters that allow stable control of oscillation wavelength and a variable-wavelength light source using such a multiple resonator.
2. Description of the Prior Art
With the advent of a broadband communication age, introduction of a WDM transmission system, which is capable of communicating by a plurality of light wavelengths in a single system, is underway aiming at more efficient use of optical fibers. Recently, a DWDM (Dense Wavelength Division Multiplexing) transmission system, which multiplexes several tens of light wavelengths to realize faster transmission, is also being widely used. This requires the WDM transmission system to be furnished with light sources for their respective light wavelengths and the number of light sources required is drastically increasing as the degree of multiplexing increases. Moreover, a ROADM (Reconfigurable Optical Add/Drop Multiplexers) system which adds/drops an arbitrary wavelength at each node is recently being introduced in inter-city communications. The ROADM system not only expands the transmission capacity through multiplexing but also changes wavelengths to allow optical paths to be switched, which increases the degree of freedom in routing within an optical network.
As a light source for a WDM transmission system, a DFB-LD (Distributed Feedback Laser Diode) which performs longitudinal single mode oscillation has been widely used so far because of its ease of use and high reliability. The DFB-LD includes a diffraction grating having a depth of approximately 30 nm formed over an entire area of a resonator, whereby stable longitudinal single mode oscillation is obtained with a wavelength corresponding to the product of the period of the diffractiong rating and double the equivalent refractive index. However, the DFB-LD cannot perform tuning extending over a wide range of oscillation wavelength. For this reason, to construct a WDM transmission system, it is necessary to use a DFB-LD product which oscillates a wavelength corresponding to each ITU grid of a defined frequency. As a result, extra stock of a variety of types of products including spares in case of malfunction needs to be provided for operation of the system, which results in an increase of shelf control cost. Moreover, with the DFB-LD, the variable-wavelength range is limited to approximately 3 nm which can be changed by a temperature variation, and therefore the actual ROADM system is constructed of a fixed-wavelength light source and a wavelength control device. For this reason, it is expected to introduce a variable-wavelength light source into the ROADM system and drastically increase the degree of freedom in wavelength control.
In order to overcome these problems with the actual DFB-LD and realize longitudinal single mode oscillation over a wide wavelength range, research into a variable-wavelength laser as a variable-wavelength light source is being vigorously conducted. Some of studies detailed in Non-Patent Document (Isao Kobayashi, “Integrated Optic Device”, first edition, second printing, KYORITSU SHUPPAN CO., LTD., December 2000, p. 104-122) will be referred and a conventional variable-wavelength laser will be explained below.
A variable-wavelength laser is largely divided into two types; one provided with a variable-wavelength mechanism inside a laser element and the other provided with a variable-wavelength mechanism outside the laser element.
As the former type, there is a proposal of a DBR-LD (Distributed Bragg Reflector Laser Diode) in which an active region producing a gain and a DBR region producing reflection by means of a diffraction grating are formed within the same laser element. The variable-wavelength range of this DBR-LD is approximately 10 nm at a maximum. There is also a proposal of a DBR-LD using a nonuniform diffraction grating in which an active region producing a gain and a DBR region which sandwiches the active region between anterior and posterior parts thereof are formed within the same laser element. In the anterior and posterior DBR regions, many reflecting peaks are produced due to the nonuniform diffraction grating and there is a slight difference in the interval of reflecting peaks between the anterior and posterior parts. This structure produces a so-called “vernier effect” providing an extremely wide variable-wavelength range. This DBR-LD using the nonuniform diffraction grating realizes variable-wavelength operation exceeding 100 nm and quasi-continuous variable-wavelength operation of 40 nm.
On the other hand, as the latter type, there is a proposal of a variable-wavelength laser which rotates a diffraction grating provided outside the laser element and returns light of a specific wavelength to the laser element.
However, though many structures are proposed for conventional variable-wavelength lasers, there are disadvantages such as a problem in securing stability called a “mode jump” that a desired wavelength is switched to an unexpected wavelength when wavelengths are switched or complicated wavelength control method, weak vibration resistance or high price due to an increase in the number of elements, and therefore the situation remains unfavorable for commercialization of such conventional variable-wavelength lasers.
The DBR-LD injects carriers into the DBR region, thereby changes a refractive index in this region and realizes variable-wavelength operation. For this reason, when crystal defects are increased by a current flow, the rate of change of the refractive index with respect to the current flow varies drastically, and therefore it is difficult to maintain laser oscillation at a constant-wavelength when used for an extended period of time. Furthermore, it is impossible to realize an “inchup” by 3 inches or more using the actual process technology of compound semiconductor. For this reason, using more complicated, large-sized laser elements will increase the price drastically.
On the other hand, in the structure with the variable-wavelength mechanism provided outside the laser element, mode jumps easily occur due to vibration, and therefore an extensive earthquake-resistant mechanism is required to avoid this, which leads to an increase in the module size and price.